1. Field of the Invention
This invention relates to a counterflow cooler for gradually reducing the temperature of processed feed pellets. The cooler advantageously provides for a gradual cooling of the pellets by accumulating a quantity of pellets in a housing and circulating air through openings in a shiftable floor through the accumulated pelletized product to minimize thermal stress in the pellets. More particularly, it is concerned with a counterflow cooler for pellets which feeds and distributes pellets into a housing until the pellets have accumulated to a specific level and are detected by a limiting sensor, thereafter cyclicly discharging a portion of the pellets through the openings while air is drawn through the openings, cooling the pellets and transferring heat to the circulating air. The pellets are cooled and gradually discharged as deflectors located over the openings permit only a portion of the pelletized product to be discharged into a hopper beneath the cooler during each cycle.
2. Description of the Prior Art
Animal feeds have long been available in processed, pelletized form. The pellets contain a wide range of ingredients and are available in a multiplicity of sizes from small pellets for feeding rabbits and the like, to dog food, and ultimately large range cubes for horses and cattle which can be from one to two inches long and three-quarters of an inch in width or diameter. After blending the ingredients and forming the pellet through a mold or by extrusion, the pellets are commonly heated or "cooked" to solidify the pellet.
The pellets leaving the processing plant are still hot, having a temperature of 160.degree. F. to 220.degree. F. If stored directly in a bin, they are subject to spoilage due to moisture accumulation and migration in the warm pellets. Moisture tends to accumulate when pellets cool over a long period, and when coupled with the warm temperatures, such protracted cooling produces spoiled pellets which are unsuitable for use.
To overcome this problem of spoilage, pellet coolers have heretofore been provided which circulate air through the pellets as they leave the production facility. One type of pellet cooler has employed a perforated conveyor which circulates air through the pellets as they are carried by the conveyor toward a hopper or bin. Another type of cooler employs a fan mounted in the bottom of a housing which draws air down through a perforated floor or grate. The pellets are fed from the top and emptied from the bottom of the grate.
Counterflow coolers, which gradually reduce the temperature of pelletized products, lessen the thermal stresses by circulating ambient air directly to a hot pellet. Counterflow coolers draw ambient air up through the pellet mass to exchange heat between the air and the pellets with the air gradually warming as it circulates into and through the pellet mass. The coolest ambient air thus circulates around the coolest pellets and accumulates heat as it moves through the pellet mass before encountering the hottest pellets. Thermal stress in the pellets is thereby minimized, resulting in fewer cracked and broken pellets.
However, to discharge the cooled pellets, some of the prior counterflow coolers relied on a plurality of gratings in which one of the grates was movable and shifted laterally relative to the other grates. The gratings of prior coolers have not been provided with an adjustable length of travel according to pellet size and have thus had difficulty in accommodating pellets of different sizes. The multiple gratings resulted in larger cubes being broken or trapped between the gratings and increased breakage of all sizes of cubes or pellets due to oscillation of the product caused by the shifting of one grate relative to the others. In addition, a more effective method of directing ventilation throughout the pellet mass was desired to ensure uniform cooling throughout the pelletized product. Prior coolers failed to provide for a uniform distribution of pellets within the cooler resulting in an uneven airflow through the pellets and an uneven rate of pellet cooling.